Surface-mount technology has several distinct advantages which are helping to foster a growing acceptance in the industry. Surface-mount components are mounted and soldered directly to a surface of a circuit board. It is unnecessary for the component terminals to penetrate the circuit board as in conventional mounting technology. This feature is especially useful in the context of electrical connectors.
The terminal members of surface-mount connectors may be soldered directly to the appropriate traces on the surface of the circuit board. In contrast, conventional connectors require drilling of multitudinous holes through the motherboard to accommodate the terminal members, each of which penetrates the motherboard and is soldered to the underside.
In eliminating the pre-drilled holes, surface-mount technology conserves board space and permits a greater trace density on each layer of the motherboard. Consequently, fewer layers are necessary. Moreover, surface-mount connectors occupy a single side of a motherboard, and there are no associated solder connections on the underside. This frees even more space because the underside is available for mounting of additional components. In addition to the economies of space, surface-mount connectors simplify the manufacturing process with fewer machining operations, relaxed tolerances, and a lower insertion force necessary for installation. In sum, considerable cost savings are made available through surface-mount technology.
Unfortunately, there have been problems encountered in the implementation of the technology. As described, conventional mounting technology incorporates terminal members which penetrate the motherboard and are soldered to the underside. These terminals inherently align the connector, and retain the connector on the motherboard during the soldering operation. Once soldered, the terminals provide an extremely strong anchor which protects the solder-joints. Surface-mount terminal members lack these inherent advantages. As a result, surface-mount connectors are commonly outfitted with one or more mounting pylons which are inserted through the motherboard to align the connector and hold it in place during the soldering operation. Once soldered, the mounting pylon(s) absorb stresses imparted to the motherboard, thereby protecting the solder-joints.
One widely used variety of surface-mount connector is the edge connector 2 shown in FIG. 1. Housing 10 of edge connector 2 is provided with a receptacle 12 adapted for receiving an edge of a daughterboard (not shown). The connector 2 is mounted directly on the surface of a motherboard 30. To help align the connector, and to retain the connector 2 during the soldering operation, connector 2 is preferably mounted on at least one support pylon 4 which extends from the bottom of the connector housing 10 at spaced intervals along the length. Support pylon 4 is locked within predrilled holes in the motherboard 30. A preferred example of a locking anchor for a support pylon can be found in related application Ser. No. 07/709,038, entitled "PYLON-ACTUATED LOCKING-EYELET", filed concurrently herewith.
Numerous electrical terminal members 20 are arranged in rows within connector housing 10 on facing sides of receptacle 12. As shown in FIG. 2, terminal members 20 are arranged in pairs of opposing terminal members. When the edge of a daughterboard is inserted within connector receptacle 12, the daughterboard is firmly retained between an opposing pair of terminal members 20.
Each terminal member 20 includes a section 24 which extends through the bottom of connector housing 10 to the surface of the motherboard 30 on which the connector is mounted. The resiliency of terminal member 20 biases it against both motherboard and daughterboard to insure a proper electrical connection therebetween.
The section of the terminal members 24 which protrudes from beneath the connector housing is known as a solder-tail. After connector 2 has been mounted on the motherboard 30, the solder-tails 24 may be conveniently soldered to adjoining traces on the surface of motherboard 30.
As shown in FIG. 2, prior art solder-tails 24 extend downward from connector housing 10 to a right-angle bend 26, at which point solder-tails 24 extend parallely across the bottom surface of connector housing 10. Hence, prior art solder-tails 24 are generally L-shaped in appearance. The L-shaped solder-tails 24 provide a minimal amount of resiliency to accommodate variations across the surface of the motherboard 30. A certain level of resiliency is necessary to insure that the numerous solder-tails 24 extending across the connector all make proper contact with motherboard 30.
However, it has been found that the resiliency of the L-shaped solder-tail of FIG. 2 is insufficient in many instances, and cannot accommodate variations in the surface of the motherboard 30. When the connector 2 is properly mounted on the motherboard 30 via support pylons 4, the resilient solder-tails 24 themselves exert a sufficient opposing normal force to contort the motherboard 30. The high normal force imparted to the motherboard 30 by a conventional surface mount solder-tail 24 will vary in accordance with the deflection of each solder-tail 24. As shown in FIG. 3 by broken line B, when the deflection of a 0.0125-inch wide by 0.0125-inch thick solder-tail 24 is approximately 1.25.times.10.sup.-3 inches, the solder-tail 24 imparts a normal force of approximately 18 grams to the surface of the motherboard 30. Therefore, the solder-tail 24 has a spring rate of approximately 14.6 grams per mil. If each terminal member of a 188-pin connector is deflected 0.004 inches, an aggregate normal force of approximately 24.2 pounds would be imparted to the surface of motherboard 30. A 0.0160-inch wide by 0.0125-inch thick solder-tail 24 would, under the above-described conditions, impart an aggregate force of approximately 30.2 pounds to motherboard 30, as shown in FIG. 3 by the solid line A.
The aggregate force imparted to the motherboard 30 may result in substantial deformation of the surface of motherboard 30, especially in the region lying between adjacent support pylons 4. For example, as shown in FIG. 4, the portion of the motherboard located midway between adjacent support pylons 4 will be distorted by an amount equal to .DELTA.y, where .DELTA.y may exceed 1 mm.
The L-shaped solder-tails 24, which themselves may partially or wholly cause non-uniformity, are unable to provide the necessary resiliency to accommodate the non-uniformity. Hence, it is likely that some solder-tails 24 will fail to make proper contact with motherboard 30. The improper contact will result in a cold or otherwise deficient solder joint when solder is later applied during the manufacturing operation.
Moreover, once the connector is mounted and soldered to the motherboard, external stresses are inevitably imparted. For example, when assembling motherboard 30 in a system, motherboard 30 may be twisted or otherwise contorted. In addition, the temperature conditions found within many electronic systems may induce temperature-related warping of the motherboard 30. Such conditions may alone or in combination cause various non-uniformities across the surface of motherboard 30, and may stress the solder-joints. Furthermore, the mating and unmating of the daughter card from the connector also puts stress on the solder joints. Conventional L-shaped solder-tails fail to provide the necessary resiliency to accommodate major variations. In addition, external stresses may break the solder-joints or solder-tails unless there is resiliency in two-directions. After mounting the connector to the board, the L-shaped solder-tails are capable only of compliancy in the vertical direction. This one-directional compliancy is insufficient to protect against stresses in the horizontal direction of the connector.
The above-described weakness of conventional surface-mount solder-tails, and the consequent risk of breakage of the terminal member or solder joint presents a reliability problem to the user, which translates into quality-assurance problems and high warranty costs for the manufacturer.
Clearly, it would be greatly advantageous to improve the compliancy of each surface-mount solder-tail to improve contact and lower the force required to mount the connector to the board, and to add compliancy in the horizontal-direction to improve the protection of the solder-joints.